Beta test: NMR TRIUMF

Ezine

Published: Aug 15, 2016

Author: David Bradley

Channels: NMR Knowledge Base

Beta decay

A depth-resolved ion-beam technique has been used by a team from The University of British Columbia (UBC) at the TRIUMF, Canada's national laboratory for particle and nuclear physics and accelerator-based science, to detect the nuclear magnetic resonance (NMR) signal from the near-surface region of an antiferromagnet.

The TRIUMF team says that magnetism is found on every scale in the instrumentation from the cyclotron's seventeen metre diameter primary magnet down to the tiny internal magnetic fields within millimetre-sized samples five orders of magnitude smaller and then further down to the samples themselves, which might be on the tens of nanometres scale and be probed with β-detected NMR.

Recently, scientists working at the facility have been quietly making breakthroughs in the magnetic characterization of nanostructures near material interfaces using β-NMR. The most recent findings from the facility have now been published in the journal Physical Review Letters and reveal a new phenomenon in the near-surface behaviour of a prototypical antiferromagnet which cannot be classified according to the standard behaviour hierarchy of such materials.

Domain alignment

David Cortie, Andrew MacFarlane and Robert Kiefl led a team from the University of British Columbia, Vancouver, Canada, and colleagues there and at the Technische Universität München, Germany and the University of Wollongong, New South Wales, Australia. "In the textbook case of an infinite 'bulk' magnet, any phase transition can be categorized as first order or second order, and there is a causal relationship between the spin dynamics and the transition temperature. Near a surface, however, the character of a phase transition is modified by broken translational symmetry," the team explains in the introduction their research.

Most common magnets are ferromagnets like iron. The internal magnetic domains are akin to minute bar magnets and can be aligned preferentially in one, north to south, direction to generate an external magnetic field. In an antiferromagnet, however, adjacent domains line-up antiparallel to each other, and so cancel out adjacent fields and thus the external field overall. Such materials do not behave like normal non-magnetic materials as a result, however, because their domains are not arranged randomly. As such, the domain ordering in an antiferromagnetic material can exhibit interesting behaviour on a small scale, especially near material interfaces. The phenomena that arise have, of course, been put to good use in the development of magnetic computer hard drives and other applications. New technologies might emerge with a deeper understanding of these phenomena.

Magnetic experiments

The team's initial work was on the archetypal antiferromagnet alpha-iron oxide, α-Fe2O3 (110), this substance is identified as an ideal test case. In the β-NMR experiments, the team dopes the near surface of the antiferromagnet material with lithium-8, 8Li+, ions at varying depths with roughly 10 nanometre precision. They then detect the βparticles, the decay electrons, which enhance the NMR signal by a factor of a billion. This provides for extreme sensitivity that allows the team to characterize the local magnetic fields on the nanometre scale, which is critical to probing surface and near-surface effects in detail.

In the current work, the team has revealed a new type of behaviour in the spin reorientation transition near the alpha-iron oxide surface that was not predicted from models of the established hierarchy of near-surface phase transitions in this substance. Nevertheless, their data showed conclusively that β-NMR can measure phase transitions, structure and dynamics near antiferromagnetic interfaces and so be used as a powerful new tool for investigating how to optimize functionality in novel nanostructures.

β-NMR experiments will receive much greater research time once TRIUMF's imminent ARIEL facility is up and running and the researchers anticipate ever more intriguing behaviour will emerge from those experiments. Specifically, the team concludes, "The observation of modified dynamics, linked to surface anisotropy, suggests a method for designing antiferromagnetic correlations in thin film heterostructures."